Recovery of pyrrole



Patented Nov. 6, 1945 2,388,475 RECOVERY OF PYRROLE Karl Henry Engel,Teaneck, N. ,L, assignor to Allied Chemical & Dye Corporation, New York,N. Y., a corporation of New York No Drawing. Application May 21', 1943,

Serial No. 487,975

3 Claims.

This invention relates to the recovery of pyrrole from mixtures ofheterocyclic nitrogen compounds in which it occurs.

'Heterocyclic nitrogen compounds such as pyrrole, pyridine and thevarious pyridine homologs such as the picolines and lutidine's, arecustomarily extracted from coke-oven distillates and other sources ofthese heterocyclic nitrogen compounds by means of an aqueous solution ofmineral acid, usually sulfuric acid. After liberation of the nitrogencompounds from their acid solutions by means of alkali, the mixtures ofnitrogen compounds are fractionally distilled. Compounds such aspyridine, boiling point 116 0., can generally be obtained in reasonablypure form by fraction-a1 distillation, but many of the remainingcompounds cannot be separated by ordinary distillation methods alone.Instead, the fractions obtained, even when they are of relatively narrowboiling range, certain increasing numbers of the various heterocyclicnitrogen compounds in each fraction.

Pyrrole occurs in admixture with picoline and other pyridine homologs ina number of natural sources of heterocyclic nitrogen compounds. Forexample, a mixture of heterocyclic nitrogen compounds recovered fromcertain coal tars contains a substantial proportion of pyrrole. Althoughpyrrolealone boils at about l29.8 C.', in admixture with heterocyclicnitrogen compounds such as picolines it forms azeotropes from which thepyrrole cannot be separated by ordinary fractional distillation. Theseazeotropes are of the high-boiling type, boiling generally within therange of about 140148 C. For example, a mixture of 20% pyrrole and 80%of a 3- and 4- picol-ine mixture in about equal proportions was found todistil through the temperature range of 146.8-147.0 0., whereas the 3-and 4"-picoline mixture alone distilled at 144A t 144.5 C. and thepyrrole alone distilled at 129.8 to 129.95 C. Upon fractionaldistillation of a mixture of heterocyclic nitrogen compounds recoveredfrom certain coal tars containing pyrrole, fractions taken within therange 140-148 0., even closecut fractions boiling for example withinabout 1 of 143 6., have been found to contain, in substantialproportions, 3-picoline (beta-picoline), boiling point 143.8 CL,4-picoline (gamma piccline), boiling point 144.8 0., generally a smallamount of 2,6-lutidine, boiling oint 143.8" C., and; in addition; asubstantial quantity of pyrrole, boiling point 129.8 C. Mixtures of thiskind have found limited practical application,

, substantial amount of pyrrole.

mainly as special solvents, but are of relatively low economic value,since picoline-lutidine mixtures containing only a few percent ofpyrrole are not water-miscible at ordinary temperatures. However, thepyrrole, which is thus detrimental in a picoline-lutidine mixture, isitself a rare chemical of great industrial value. For example, pyrrolederivatives are of importance as pharmaceuticals, agriculturalchemicals, e. g., insecticides, perfume bases, and as antiseptics, andit is only the high cost of preparing pyrrole by the complicatedprocedures now in use that limits its employment in these and otherfields. I,

Since the components making up the heterocyclic compound fractionscontaining pyrrole form constant-boiling azeotropes, fractionaldistillation is obviously unsatisfactory as a means for resolving themixtures. Methods heretofore proposed in the published prior art forseparating pyrrole, from such mixtures havebeen generally unsatisfactoryin products, yields, and production costs. One method proposed for suchseparation, for example, is to treat the heterocyclic compound mixturewith an excess of sulfuric acid to hold the picolines and other pyridinehomologs in combination and subject the resulting mixture to prolongedsteam distillation to remove the pyrrole which is only loosely held inthe sulfuric acid-pyridine homolog mixture, This method has been foundunsatisfactory, however, since heating in the presence of sulfuric acidconverts pyrrole into dark polymeric substances insoluble in hydrocarbonoils, nitrogen bases or water, forming a jelly-like precipitate which itis impractical to separate, and when the pyridine homolog sulfates arereconverted to free nitrogen compounds by treatment with aqueous alkali,they are found to be still in admixture with a This method is thusineffective for separation of pyrrole from pyridine homologs and,moreover; a large part of the valuable pyrrole is converted into aproduct of little or no utility. It has further been proposed to recoverpyrrole and its homologs from mixtures thereof with aromatichydrocarbons recovered from organic degradation products, by treatingthe mixtures with potassium hydroxide to form potassium pyrroles. It hasbeen found, however, that even when a large excess of solid potassiumhydroxide is employed over the stoichiometric amount required to formpotassium pyrroles in this prior art process, only a small proportion ofthe py roles can be separated in the form of their potassium compounds.The yield and the production cost in this method as practised in theprior art have been found so unfavorable that the method is notconsidered feasible for industrial use.

It is an object of this invention to'provide a process for separatingsubstantially pure pyrrole from admixture with other heterocycliccompounds not readily separable therefrom by distillation.

It is a further object of this invention to provide a commerciallyattractive method for recovering pyrrole from fractions of heterocyclicnitrogen compounds, particularly of coke-oven origin, boiling in therange from 140-148 C., which fractions contain 3-picoline, 4-picolineand pyrrole in substantial proportions, and may contain 2,6-lutidine.

I have now discovered that pyrrole may be precipitated in the form ofpotassium pyrrole in surprisingly good yield and high purity frommixtures of pyrrole with other heterocyclic nitrogen compounds notseparable from pyrrole by distillation, e. g., such other compounds as3-picoline, 4-picoline and 2,6lutidine, by making and maintaining thereaction mixture in which the potassium pyrrole is formed substantiallycompletely anhydrous. The process of the invention thus involvesremoving from the reaction mixture substantially all water formed by thereaction while treating a pyrrole-containing heterocyclic nitrogencompound mixture, as above described, with potassium hydroxide toconvert the pyrrole to potassium pyrrole. Under these conditions I havefound that up to 85% of the pyrrole may be precipitated in the form ofpotassium pyrrole, even though no more than the theoretical amount ofpotassium hydroxide is employed for the reaction. The removal of waterof reaction from a pyrrole-containing mixture treated with potassiumhydroxide, I have found, may advantageously be effected by refluxing thereaction mixture with a water-entraining agent such as a hydrocarbon oilof coal-tar or petroleum origin, 1. e. an aromatic, alicyclic, oraliphatic hydrocarbon of saturated or unsaturated character and boilingbelow the boiling point of the nitrogen compounds present, e. g.,boiling below about 140 0., preferably in the range from about 80-140 C.

The process of my invention may be advantageously applied to therecovery of pyrrole from heterocyclic nitrogen compound mixturesobtained from coke-oven distillates, cracked petroleum, shale tar, boneoils, the products of de-- structive distillation of proteinaceousmaterials and from other natural, as well as synthetic, sources whenthese mixtures contain substantial amounts of pyrrole and other nitrogencompounds not readily separable therefrom by distillation, e. g.,3-picoline, 4-picoline and 2,6- lutidine. When such a mixture ofheterocyclic nitrogen compounds contains components of widely varyingboiling point, the mixture is first subjected to fractional distillationwhereby a fraction boiling in the range 140148 C. containingpredominantly pyrrole and other nitrogen compounds not separabletherefrom by ordi nary distillation is obtained. My invention comprisesa process in which such a fractionation is combined with theabove-described separation of pyrrole in the form of potassium pyrrole.

My invention further comprises a process in which separation of pyrroleis combined with steps for isolation of the other heterocyclic nitrogencompounds present with pyrrole in these close cut fractions. Thus myinvention is applicable to resolving into its components a mixture ofpyrrole, 3-picoline, 4-picoline and 2,6-lutidine.

In a preferred method of carrying out a process of my invention, apyrrole fraction, as above described, suitably a fraction boiling in therange 140-148 C. containing pyrrole, B-picoline, 4- picoline, and smallamounts of 2,6-lutidine, is

- mixed with approximately the calculated quantity of potassiumhydroxide in either solid or concentrated aqueous solution form forreaction with pyrrole, in accordance with the equation A smallproportion of a hydrocarbon oil such as benzene or toluene, or apetroleum fraction boiling below about 140 C., is added to the mixtureof nitrogen compounds, unless such hydrocarbons are naturally present inthe nitrogen compound mixture which may be the case when a pyrrolesource such as a low-boiling coal-tar oil or other natural pyrrole crudeis treated. Preferablman amount of hydrocarbon oil is employed such thatit constitutes from 5% to 20% of the mixture. A mixture thus obtainedwith the added or naturally present hydrocarbons is agitated and heatedto refluxing. Refluxing condensate consisting mainly of hydrocarbon oilswith entrained water is not returned directly to the reaction charge,but is by-passed into a settling device where it separates into an upperlayer of hydrocarbon oils, including dissolved nitrogenous compounds,and a lower Water layer. The upper layer is continuously orintermittently returned to the still charge. The water layer is removedand the revfiuxing is continued until all the reaction water and anyadditional water originally present as moisture in the charge has beenwithdrawn.

The desired reaction proceeds smoothly under these conditions. Potassiumpyrrole is soluble to some extent in the hot hydrocarbon-nitrogencompound mixture, but begins to crystallize in the course of thedehydration in small but wellformed granular crystals. The solubility ofthe potassium pyrrole in anhydrous cold solutions of reaction mixture isvery small. The reaction mixture is cooled and potassium pyrrolecrystals are isolated by filtration, e. g., centrifuging. The crystalsmay be washed with a little hydrocarbon solvent to remove adhering oilsand may be dried by heating to 95 C. under reduced pressure.

Potassium pyrrole is, itself, a useful product: it may be employed inthe preparation of various pyrrole derivatives, e. g. proline, whichhave varied uses in pharmaceutical and other fields. The potassiumpyrrole is also a most advantageous source for substantially purepyrrole. Pyrrole is readily liberated from the potassium pyrrole by theaddition of water, the resulting mixture settling to a lower layer ofpotassium hydroxide solution and an upper layer of pyrrole. The pyrrolelayer is decanted, dried, and distilled, giving a product of 99% to99.5% purity. Compounds such as picolines present, even in amount lessthan 1% in the pyrrole product, may be removed by extracting thematerial with a. small quantity of dilute oxalic acid solution andthereafter washing with water. A pyrrole product of substantially 100%purity may thus be obtained. The aqueous lower layer of potassiumhydroxide solution may be cyclicly reused in the process, preferablyafter being dehydrated to some extent by free evaporation.

The mother liquor remaining after filtration of the potassium pyrrolecrystals may be treated to recover a pyridine homolog mixture. This maygenerally be accomplished merely by free.

tional distillation; however, if necessary, the mother liquor may bewashed with sulfuric acid to extract the pyridine homologs which maythen be freed from their sulfate solution by addition of aqueous alkali.The pyridine homog mixture thus recovered, generall containing3-picoline, 4-picoline, and 2,6-lutidine, may be resolved into itscomponents by methods described in copending applications, e. g:, themethod of my copending application Serial No. 456,257, filed August 26,1942, or the methods of copending applications Serial'No. 441,557, filedMay 2, 1942, and Serial No. 452,369, filed July 25, 1942.

The following example is illustrative of the process of my invention:

An iron kettle, equipped for agitation and supplied with a refluxcondenser and settling trap for collecting condensate, Was charged asfollows:

1500 parts by weight of a mixture of heterocyclic nitrogen compoundshaving approximately the following composition:

Per cent Pyrrole 19 3-pico1ine 42 4-picoline (approximately) 33 and theremainder consisting of 2,6-lutidine and traces of 2,4-lutidine.

300 parts by weight of potassium hydroxide, in pellet form, containingapproximately 255 parts by weight of pure potassium hydroxide,

215 parts by weight of toluene.

The reaction mixture was agitated and heated to refluxing, the inside ofthe kettle attaining a temperature of 130 C. A total of 95 parts ofwater was removed over a period of 16 hours of refluxing. The kettlecharge was cooled to C. and the crystalline potassium pyrrole isolatedby filtration, Crystals were washed with benzene and dried at 90-95 C.,438 parts by weight of dry material being obtained.

Potassium pyrrole Was decomposed by addi- -tion of 350 parts of water,the resulting mixture being allowed to settle in a lower layer ofpotassium hydroxide solution and an upper layer of pyrrole. The isolatedpyrrole was found to contain 0.6% by weight of picolines. These wereremoved by extracting the material with a small quantity of diluteoxalic acid solution and with water. The product, on a moisture-freebasis, amounted to 225 parts by weight or about 78% of the pyrrolepresent in the crude mixture, It was distilled for color and found tohave the following characteristics:

of potassium pyrrole crystals was separated by fractional distillationinto toluene and pyridine bases. The latter contained from 2% to 3% ofresidual pyrrole.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

I claim:

1. A process for recovering pyrrole from a mixture of heterocyclicnitrogen compounds from coal tar, containing pyrrole along with pyridinehomologs including a picoline selected from the group consisting of3-picoline and 4- picoline, which process comprises fractionating saidmixture to obtain a fraction boiling in the range of about 140- 48 C.,treating sa d fraction with potassium hydroxide in amount sufilcient toreact with the pyrrole to form potassium pyrrole, refluxing theresulting mixture together with a small proportion of hydrocarbon oilboiling below about 140 C., with separation of water from the refluxcondensate, until substantially all Water has been removed from thereaction mixture, and separating the potassium pyrrole whichprecipitates.

2. A process for recovering pyrrole from mixtures of pyrrole with3-picoline and 4-picoline, which comprises treating the mixture withpotassium hydroxide in substantially the stoichiometric amount to reactwith the pyrrole to form potassium pyrrole, refluxing the resultingmixture together with about 5% to 20% by weight, based on the totalmixture, of hydrocarbon oil boiling in the range of about 80 to 140 C.,with separation of water from the reflux condensate, until substantiallyall water has been removed from the reaction mixture, cooling theresulting substantially anhydrous reaction mixture, separating thepotassium pyrrole which precipitates, and reconverting said potassiumpyrrole to pyrrole b treatment with Water.

3. A process for recovering pyrrole from a mix- .ture of heterocyclicnitrogen compounds con- Specific gravity at 25/4 C. -4- 0.9691Refractive index 'n at 25 C 1.5095 True boiling point 129.8 C.,corrected Odor Ethereal, resembling chloroform The material waswater-white when freshly distilled.

The mother liquor remaining after isolation taining pyrrole along withpyridine homologs including a picoline selected from the groupconsisting of 3-picoline and 4-picoline, which process comprisesfractionating said mixture to obtain a fraction boiling in the range ofabout -148 C., treating said fraction with potassium hydroxide in amountsufficient to react with the pyrrole to form potassium pyrrole,refluxing the resulting mixture together with a small proportion ofhydrocarbon oil boiling below about 140 C., with separation of waterfrom the reflux condensate, until substantially all water has beenremoved from the reaction mixture, and separating the potassium pyrrolewhich precipitates.

KARL HENRY ENGEL.

